Vessel segmentation for time resolved phase contrast MRI

Quantification of cardiovascular flow and blood volumes is a useful tool in diagnosing car-diovascular disease such as congenital heart defects and different kinds of valve leakage. Medical imaging techniques enables non-invasive analysis of anatomy and physiology. In order to perform flow quantification from medical image sequences, the boundaries of the vessels of interest are usually delineated manually by medical professionals. This is a time consuming process and the result depends, to a high degree, on user experience. This thesis presents an automated vessel segmentation method for the main vessels around the heart from velocity encoded Magnetic Resonance Imaging sequences. The proposed method only require a manual delineation in one image. The algorithm is based on an active contour, using the Euler-Lagrange equation together with internal and external forces designed from a set of fundamental assumptions regarding vessel shape and behaviour. More specifically, constraints were applied to the geometrical shape and elasticity of a vessel. Validation of the method was performed by comparing the detected stroke volume with manual delineation, and also by measuring the segmentation overlapping of the two methods. On a test set of 20 patients, 19 resulted in excellent segmentation agreement with manual delineations, with a mean Dice coefficient over 0.8. However, performance instability was observed when chang-ing the values of two algorithm parameters, and one of the patients in the set constantly resulted in segmentation failure for all tested parameter combinations. The relative variabil-ity in stroke volume between the proposed algorithm and manual delineation was, at best, 6 ± 3.6%. This is comparable to the interobserver variability from a previous physiological study [1] of 3 ± 4%, which indicates the potential of the suggested method if improvements in robustness and stability is implemented.